[Python-ideas] The async API of the future: yield-from

[Terry Reedy]

On 10/14/2012 1:42 PM, Guido van Rossum wrote:
 > On Sun, Oct 14, 2012 at 10:27 AM, Terry Reedy <tjreedy at udel.edu> wrote:
 >> On 10/14/2012 10:36 AM, Guido van Rossum wrote:
 >>> Of course there's the question of what to do when one of the tasks
 >>> raises an error -- I haven't quite figured that out in NDB either, it
 >>> runs all the tasks to completion but the caller only sees the first
 >>> exception. I briefly considered having an "multi-exception" but it
 >>> felt too weird -- though I'm not married to that decision.

 >> One answer is to append the exception object to results and let the
 >> requesting code sort out what to do.
 >>
 >>
 >> def par(*args):
 >>     results = []
 >>     for task in args:
 >>        try:
 >>
 >>           result = yield from task
 >>           results.append(result)
 >>        except Exception as exc:
 >>           results.append(exc)
 >>     return results
 >
 > But then the caller would have to sort through the results and check
 > for exceptions. I want the caller to be able to use try/except as
 > well.

OK. Then ...

   def par(*args):
      results = []
      exceptions = False
      for task in args:
         try:
            result = yield from task
            results.append(result)
         except Exception as exc:
            results.append(exc)
            exceptions = True
      if not exceptions:
         return results
      else:
         exc = MultiXException()
         exc.results = results
         raise exc

Is this is what you meant by 'multi-exception'?

caller:

try:
    results = <whatever>
    <process results, perhaps by iterating thru them, knowing all 
represent successed>
except MultiXException as exc:
    errprocess(exc.results)